In view of recent environmental problems, there has been growing expectation for fuel cells, particularly for polymer electrolyte fuel cells using a proton conductive polymer electrolyte membrane because the fuel cells can be operated at low temperatures and have a possibility of reduction in size and weight. As the polymer electrolyte used for the polymer electrolyte fuel cells, there is known a fluoro-polymer having superstrong acid groups that is represented by, for example, “Nafion” (Nafion is a registered trademark of DuPont, hereinafter the same). However, the fluoro-polymer having superstrong acid groups is extremely expensive since it is a fluoro-polymer, and also the fluoro-polymer has a relatively low heat resistance, a high alcohol permeability, and a disadvantage that environmental consideration is necessary on preparation and disposing of the fluoro-polymer.
In order to address the disadvantages that the fluoro-polymer having superstrong acid groups is expensive, and has relatively low heat-resistance and high alcohol permeability, a number of proposals have been already submitted for a polymer electrolyte membrane based on a less expensive non-fluoro polymer. In particular, in view of durability, heat resistance, and cost of the polymer electrolyte membrane, a hydrocarbon aromatic polymer electrolyte is desirable. As the polymer electrolyte membrane, there are disclosed, for example, in Japanese Patent Application Laid-Open No. H11-502249 (Patent Document 1), Japanese Patent Application Laid-Open No. 2002-524631 (Patent Document 2) and others, a sulfonated polyetherketone polymer electrolyte membrane; in Japanese Patent Application Laid-Open No. 2000-510511 (Patent Document 3), Japanese Patent Laid-Open Publication No. 2003-64181 (Patent Document 4), Japanese Patent Laid-Open Publication No. 2003-68326 (Patent Document 5) and others, a sulfonated polyimide polymer electrolyte membrane; in U.S. Pat. No. 5,403,675 (Patent Document 6) and others, a polyarylene polymer electrolyte membrane; and in Japanese Patent Laid-Open Publication No. H10-21943 (Patent Document 7), Japanese Patent Laid-Open Publication No. H10-45913 (Patent Document 8), Japanese Patent Laid-Open Publication No. H11-116679 (Patent Document 9) and others, a sulfonated polyethersulfone polymer electrolyte membrane. Further, Japanese Patent Laid-Open Publication No. H11-67224 (Patent Document 10) and others disclose a membrane/electrode assembly using a sulfonated polyethersulfone polymer electrolyte membrane.
The improvement of properties on water-absorption has been requested, and a block copolymer has been proposed as the means for solving the problem. For example, there are disclosed, in Japanese Patent Laid-Open Publication No. 2002-60687 (Patent Document 11), Japanese Patent Laid-Open Publication No. 2004-190002 (Patent Document 12), Japanese Patent Laid-Open Publication No. 2004-190003 (Patent Document 13), Japanese Patent Laid-Open Publication No. 2004-346305 (Patent Document 14) and others, polyether; and in Japanese Patent Laid-Open Publication No. 2002-358978 (Patent Document 15), Japanese Patent Laid-Open Publication No. 2003-234014 (Patent Document 16), Japanese Patent Application Laid-Open No. 2003-511510 (Patent Document 17) and others, polyimide. Further, Japanese Patent Laid-Open Publication No. 2003-31232 (Patent Document 18) discloses a polyether sulfone block copolymer that has an improved temperature dependence of proton conductivity. However, there is not any description on the relationship between the ion exchange capacity and proton conductivity of the hydrophilic segment of the block copolymers in these Patent Documents. Still further, Japanese Patent Laid-Open Publication No. 2005-126684 (Patent Document 19) and Japanese Patent Laid-Open Publication No. 2005-139432 (Patent Document 20) also disclose a sulfonated polyether block copolymer, but there are not any descriptions about the proton conductivity under low-humidity conditions, also there are not any specific descriptions about the membrane properties on water-absorption.
As the above-mentioned block copolymers, particularly a block copolymer exemplified in Japanese Patent Laid-Open Publication No. 2003-31232 (Patent Document 18) and others is desirable from the viewpoint of heat-resistance, durability and cost, since the block copolymer has a polyarylether sulfone type or a polyaryletherketone type on both segments, having no specific linking group that connects both segments, and is synthesized not through a reaction using a metal complex such as coupling reaction. In particular, it is desirable because of low cost that the block copolymer is prepared as follows: a prepolymer serving as a hydrophilic segment is synthesized using a dichloride compound having sulfonic acid groups as a source material; and then the prepolymer and a hydrophobic segment prepolymer that is synthesized separately are reacted through ether-exchange reaction.
Japanese Patent Laid-Open Publication No. 2003-206354 (Patent Document 21) discloses the production method of a polyarylethersulfone block copolymer using ether-exchange reaction. However, this document describes about a hydrophilic segment prepolymer having no sulfonic acid groups. There are not any specific descriptions about the method of synthesizing the block copolymer through ether-exchange reaction using a hydrophilic segment prepolymer that has sulfonic acid groups
As the dihalide compound having sulfonic acid groups that is used for the synthesis of polyarylethersulfone or polyaryletherketone that has sulfonic acid groups, there are mainly used a dihalide compound that contains fluorine and chlorine as halogen element. However, the dihalide compound that has sulfonic acid groups and contains fluorine as halogen element is expensive. For the synthesis of sulfonated polyarylethersulfone, in most cases, as the alkali metal salt of the dichloride having sulfonic acid groups, a sodium salt has been specifically used so far. However, the problem is that the block copolymer is hardly synthesized through ether-exchange reaction even though a hydrophilic segment prepolymer having a sodium salt as a sulfonate group is synthesized using the sodium salt of the dichloride having sulfonic acid groups, and then block-copolymerized using the synthesis.
The polymer electrolyte membrane described above is produced in an industrial process as follows: a polymer electrolyte solution is continuously cast on a support and heated so as to evaporate the solvent until a self-supporting membrane is formed; and then the self-supporting membrane is peeled off from the support. As the support, a belt of a metal such as stainless steel is used in general. The polymer electrolyte membrane having strong acid groups or superstrong acid groups is not easy to peel off as a self-supporting membrane from the metal belt such as a stainless steel belt, and the membrane is sometimes broken or scared during peeling off. Use of phosphate ester and/or salt between phosphate ester and amine as a separating agent is disclosed in the method for adding to a solution of polyamide acid that is a polyimide precursor in Japanese Patent Laid-Open Publication No. S60-244507 (Patent Document 22). However, there are not any descriptions about the production of the polymer electrolyte membrane having strong acid groups such as sulfonic acid or phosphoric acid or superstrong acid groups such as fluorinated alkylsulfonic acid.
Fuel cells installed in vehicles such as automobiles are required to be small in weight and volume and cannot be attached with a large humidification apparatus. Therefore, high proton conductivity is required under low humidity conditions. To improve the proton conductivity, increase of the ion-exchange capacity is effective, but to the contrary there was a problem that the membrane properties on water-absorption are lowered. In this way, a membrane having still higher proton conductivity has been requested among the membranes that have the same ion-exchange capacity.
When the ion-exchange capacity is increased so as to maintain adequate proton conductivity, the membrane properties on water-absorption is lowered. As a measure, a blend with a non-sulfonated polymer has been proposed. For example, in Japanese Patent Laid-Open Publication No. H08-20716 (Patent Document 23), there is disclosed a composition that is composed of sulfonated polyetherketone, polysulfone and a hydrophilic polymer; and also in Japanese Patent Laid-Open Publication No. 2002-260687 (Patent Document 24), there is disclosed a composition that is composed of polyimide and a block copolymer having sulfonic acid groups. Further, in order to improve the adhesion to electrode layers, a composition that is composed of an aromatic polyether and a sulfonated aromatic polyether and has a flow start temperature of from 100° C. to 220° C. is disclosed in Japanese Patent Laid-Open Publication No. 2004-307629 (Patent Document 25). However, in the case of a blend of a sulfonated homo-polymer or a random copolymer with a non-sulfonated polymer, and a blend of a block copolymer having sulfonic acid groups with a non-sulfonated polymer that does not have the similar structure unit to the block copolymer, when the non-sulfonated polymer is blended to an extent at which improved membrane properties on water-absorption are attained, the proton conductivity is largely lowered at low humidity although there are not any problems on the proton conductivity at high humidity. On the other hand, when the non-sulfonated polymer is blended in an amount at which the proton conductivity at low humidity is not lowered largely, there was a problem that the membrane properties on water-absorption was not sufficiently improved.    Patent Document 1: Japanese Patent Application Laid-Open No. H11-502249    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-524631    Patent Document 3: Japanese Patent Application Laid-Open No. 2000-510511    Patent Document 4: Japanese Patent Laid-Open Publication No. 2003-64181    Patent Document 5: Japanese Patent Laid-Open Publication No. 2003-68326    Patent Document 6: U.S. Pat. No. 5,403,675    Patent Document 7: Japanese Patent Laid-Open Publication No. H10-21943    Patent Document 8: Japanese Patent Laid-Open Publication No. H10-45913    Patent Document 9: Japanese Patent Laid-Open Publication No. H11-116679    Patent Document 10: Japanese Patent Laid-Open Publication No. H11-67224    Patent Document 11: Japanese Patent Laid-Open Publication No. 2002-60687    Patent Document 12: Japanese Patent Laid-Open Publication No. 2004-190002    Patent Document 13: Japanese Patent Laid-Open Publication No. 2004-190003    Patent Document 14: Japanese Patent Laid-Open Publication No. 2004-346305    Patent Document 15: Japanese Patent Laid-Open Publication No. 2002-358978    Patent Document 16: Japanese Patent Laid-Open Publication No. 2003-234014    Patent Document 17: Japanese Patent Application Laid-Open No. 2003-511510    Patent Document 18: Japanese Patent Laid-Open Publication No. 2003-31232    Patent Document 19: Japanese Patent Laid-Open Publication No. 2005-126684    Patent Document 20: Japanese Patent Laid-Open Publication No. 2005-139432    Patent Document 21: Japanese Patent Laid-Open Publication No. 2003-206354    Patent Document 22: Japanese Patent Laid-Open Publication No. S60-244507    Patent Document 23: Japanese Patent Laid-Open Publication No. H08-20716    Patent Document 24: Japanese Patent Laid-Open Publication No. 2002-260687    Patent Document 25: Japanese Patent Laid-Open Publication No. 2004-307629